Process for phosphating steel strip galvanized on one side

ABSTRACT

A process for phosphating a galvanized surface of a steel strip electrolytically galvanized on one side by contacting the galvanized surface of the steel strip with a phosphating solution containing 1.0 to 6 g/l zinc, 0.5 to 5.0 g/l nickel and 14 to 25 g/l phosphate at a temperature of 40° to 70° C. for from 2 to 20 seconds. The ungalvanized surface remains metal bright.

FIELD OF THE INVENTION

This invention relates to a process for phosphating steel stripelectrolytically galvanized on one side to form nickel-containing zincphosphate coatings on the galvanized surface. The zinc phosphatecoatings are applied by brief spraying with or immersion in aqueousacidic phosphating solutions.

RELATED ART

Processes for phosphating surfaces of iron, steel, zinc and alloysthereof and also aluminium have long been known (Ullmann's Encyklopadieder technischen Chemie, 4th Edition, Vol. 15, pages 686 and 687). Theobject of phosphating the surfaces in question is to increase theadhesion of paint coatings and to improve protection against corrosion.

DE-A-32 45 411 describes a process for phosphating electrolyticallygalvanized metal articles, more particularly electrolytically galvanizedsteel strip, by brief treatment with acidic phosphating solutions which,besides zinc and phosphate ions, may contain other metal cations and/oranions of oxygen-containing acids with an accelerating effect. Zincphosphate coatings with a weight per unit area of less than 2 gm⁻² areformed in this process. The acidic phosphating solutions used have acontent of Zn²⁺ cations of around 1 to 2.5 g/l and a free acid contentof 0.8 to 3 points, the acid ratio of total acid to free acid being keptin the range from 5 to 10. The treatment is said to last not much longerthan 5 seconds. Nitrate-containing phosphating baths are preferablyused, the ratio by weight of Zn²⁺ to NO₃₋ being in the range from 1:1 to1:8 and the ratio by weight of PO₄ ³⁻ to NO₃₋ being in the range from1:0.1 to 1:2.5.

EP-A-0 219 779 describes a process for phosphating electrolyticallygalvanized metal articles, preferably electrolytically galvanized steelarticles, more particularly electrically galvanized steel strip, bybrief treatment for not much longer than 5 seconds with acidicphosphating solutions which, besides zinc, manganese and phosphate ions,may contain other metal cations and/or anions of oxygen-containing acidswith an accelerating effect. To produce coatings consistingpredominantly of zinc phosphate with a weight per unit area of less than2 gm⁻², the phosphating solutions used have a content of zinc cations(Zn²⁺) of 0.1 to 0.8 g/l, a content of manganese cations (Mn²⁺) of 0.5to 2.0 g/l and a free acid content of 4 to 8 points for an acid ratio(total acid to free acid) of 2.5 to 5.

JP-A-62 020 879 describes a process for phosphating galvanized steelstrip on one side. In this process, phosphating is carried out byspraying of the phosphating solution onto the underneath of the steelstrip while compressed air is blown onto the upper surface of the steelstrip.

Now, the problem addressed by the present invention was to produce zincphosphate coatings by spray or dip application to steel stripelectrolytically galvanized on one side in such a way that, even afterphosphating, the steel side would still represent a metallically brightsurface. According to the invention, this would be done in treatmenttimes of 2 to 20 seconds without any deterioration in corrosionprotection and of course in such a way that dense, compact phosphatecoatings would be formed on the galvanized side in the treatment timesmentioned and in such a way that satisfactory forming properties wouldbe obtained.

A further problem addressed by the invention was to apply the phosphatecoatings thinly with weights per unit area of 0.5 to 3.0 gm⁻² and, moreparticularly, in the range from 1.0 to 2.0 gm⁻² without at the same timehaving to sacrifice uniform coverage of the steel strip galvanized onone side with a finely crystalline, firmly adhering and compact zincphosphate coating.

In the context of the invention, the expression "steel stripelectrolytically galvanized on one side" is of course also intended toencompass generally known zinc alloys (for example ZNE-electrolyticallyapplied zinc alloy containing 10 to 13% of Ni or ZFE-electrolyticallyapplied, Fe-containing zinc alloy).

The problems stated above have been solved by a process for phosphatingthe galvanized surface of steel strip electrolytically galvanized on oneside to form nickel-containing zinc phosphate coatings by brief sprayingwith or immersion in acidic phosphating solutions, the treatment timebeing 2 to 20 s, depending on the rate of travel of the strip, thephosphating process being carried out at temperatures of 40° to 70° C.and the phosphating solutions satisfying the following requirements:

content of Zn²⁺ cations: 1.0 to 6.0 g/l,

content of Ni²⁺ cations: 0.5 to 5.0 g/l,

content of PO₄ ³⁻ anions: 14 to 25 g/l,

"free acid" content: 3.5 to 0.0 points,

"total acid" content: 22 to 40 points.

The parameters mentioned above and their determination are described indetail in Chr. Ries, "Uberwachung von Phosphatierungsbadern (Monitoringof Phosphating Baths)", Galvanotechnik, 59 (1968), No. 1, pages 37-39(Eugene. Leuze-Verlag, Saulgau). Accordingly, the free acid content inpoints is defined as the quantity of 0.1N NaOH in ml needed to titrate10 ml of bath solution against dimethyl yellow, methyl orange orbromphenol blue. The total acid content in points is defined as thequantity of 0.1N NaOH in ml needed to produce the first pink colorationin the titration of 10 ml of bath solution using phenolphthalein asindicator.

Accordingly, the combination of all the parameters mentioned is crucialto the process according to the invention.

BRIEF SUMMARY OF THE INVENTION

It is known that zinc is quickly dissolved out from the galvanized stripby the acidic phosphating solutions. Under certain plant operatingconditions, a relatively high zinc content is established in thephosphating bath through the usual introduction of Zn²⁺ cations by thesteel strip galvanized on one side. Due to the establishment ofequilibrium, a content of Zn²⁺ cations of 3.0 to 5.0 g/l is preferredfor the purposes of the invention.

If the content of nickel cations is less than 0.5 g/l, adhesion betweenthe substrate and the coating after cataphoresis is inadequate. On theother hand, a nickel content of more than 5.0 g/l does not produce anyfurther improvements so far as subsequent coating is preferred.Accordingly, the content of nickel cations is preferably adjusted insuch a way that a concentration of 1.0 to 3.0 g/l is maintained in thephosphating bath.

DETAILED DESCRIPTION OF THE INVENTION

If the content of phosphate anions in the solution is less than 14 g/l,a defective zinc phosphate coating is formed. On the other hand, aphosphate content of more than 25 g/l does not provide for anyadditional advantageous effects. Accordingly, the use of relativelylarge amounts of phosphate has economic disadvantages. Accordingly, itis preferred to use a phosphating solution containing 19 to 23 g/l ofphosphate ions.

The simultaneous presence of nickel cations and other divalent cationscan improve paint adhesion, depending on the paint systems used, and canincrease the corrosion protection level of the system after painting. Ina preferred embodiment of the invention, therefore, the phosphatingsolutions additionally contain at least one other divalent cationselected from manganese, cobalt, iron, calcium and magnesium. Accordingto the invention, these additional divalent cations are preferably usedin the following concentrations in the phosphating solutions: 0.2 to 3.0g/l manganese, 0.5 to 4.0 g/l cobalt, 0.05 to 1.0 g/l iron and 0.5 to5.0 g/l calcium or magnesium. Divalent manganese is preferably used asthe additional cation.

According to the invention, a crucial criterion is the time required forthe phosphating treatment. Whereas, in the automotive industry, timesof >120 seconds are normally required for phosphating, the target timein the phosphating of steel strip galvanized on one side is always wellbelow 1 minute. According to the present invention, therefore, thetreatment time is between 2 and 20 seconds and preferably between 5 and10 seconds.

The major advantage of the present invention is that it enables zincphosphate coatings to be produced on steel strip galvanized on one sidein such a way that the galvanized surface is light in appearance whilethe ungalvanized surface remains metallically bright.

Although the presence of fluoride anions is generally not necessary inthe case of steel strip electrolytically galvanized on one side, it canbe of advantage in the interests of uniform coating. Accordingly,another preferred embodiment of the present invention is characterizedin that the phosphating solutions have a content of fluoride anions of0.0 to 0.5 g/l and preferably 0.1 to 0.2 g/l.

Phosphating itself is carried out at moderately elevated temperatures ofaround 40° to 70° C. Temperatures in the range from 55° to 65° C. can beparticularly suitable.

The preparation of the phosphating solutions for carrying out theprocess according to the invention is generally effected in the usualmanner known per se to the expert. Thus, water-soluble salts of thecations mentioned, for example the nitrates or carbonates, orcorresponding soluble oxides or hydroxides may be used as startingmaterials for the preparation of the phosphating solutions. In general,the cations mentioned are used in the form of their nitrates althoughthe nitrate content of the phosphating solutions has no effect on thephosphating result. The phosphate anions are generally introduced intothe phosphating solutions in the form of phosphoric acid. The total acidcontent is adjusted through the content of phosphate anions in thephosphating solutions. The free acid content is optionally adjusted byincreasing the pH value, for example by addition of sodium hydroxide orsodium carbonate. The fluoride anions optionally used in the phosphatingsolutions are generally introduced in the form of sodium fluoride orcomplex fluorides, for example tetrafluoroborate or hexafluorosilicate.

Before the phosphating solution is applied, the surface electrolyticallygalvanized on one side must be completely wettable with water. This isgenerally the case in continuous strip plants. If the surface of thestrip galvanized one side has been oiled for storage and corrosionprevention, the oil must be removed before phosphating by suitable,already known preparations and methods. Thereafter the water-wettablegalvanized metal surface is best subjected to an activating pretreatmentknown per se before the phosphating solution is applied. Suitablepretreatment methods are described in particular in DE-A-20 38 105 andin DE-A-20 43 085. Accordingly, the metal surfaces to be subsequentlyphosphated are treated with solutions essentially containing titaniumsalts and sodium phosphate, optionally together with organic components,such as alkyl phosphonates or polycarboxylic acids for example, asactivating agents. Soluble compounds of titanium, such as potassiumtitanium fluoride or titanyl sulfate, may advantageously be used as thetitanium component. Disodium orthophosphate is generally used as thesodium phosphate.

As described in the prior art (for example in DE-A-32 45 411), it canalso be of advantage for the process according to the invention orrather the zinc phosphate coatings produced by the process according tothe invention to passivate the phosphate layers produced in a followingprocess step. The passivating treatment may be carried out, for example,with solutions containing chromates and Cr(III) salts or with Cr-freesolutions.

Zinc phosphate coatings with a weight per unit area of less than 2 gm⁻²are produced on the galvanized side by the process according to theinvention. The zinc phosphate coatings have a compact, finelycrystalline structure and provide the steel strip electrolyticallygalvanized on one side with a desirable, uniform light grey appearance,the non-galvanized side being left with a metallically bright surface. Asteel strip phosphated in this way may be further processed even withoutsubsequent painting. The thin phosphate coatings produced by the processaccording to the invention show favorable behavior in numerous formingprocesses. Even subsequently applied organic coatings show distinctlyimproved adhesion in relation to the prior art both during and after theforming processes.

EXAMPLES

Steel electrolytically galvanized on one side (2.5 to 7.5 μm Zn) wassurface-treated by the usual sequence of process steps:

1. Cleaning and degreasing

Spray application of surfactant-containing alkaline cleaners (such asRIDOLINE® C 72) for 5 to 20 seconds at 50° to 60° C.

2. Rinsing

3. Activation

Spray application of preparations containing titanium salts (such asFIXODINE® 950) for 2 to 4 seconds at 20° to 40° C.

4. Phosphating

For composition, see Table 1.

5. Rinsing

6. After-passivation

Spray or dip application of chromium-containing after-passivatingpreparations (such as DEOXYLYTE® 41B) for 2 to 6 seconds at 20° to 50°C.

7. Squeezing

Surplus liquid is removed by squeezing rollers without compression ofthe layer.

8. Drying

The strip dries under its own heat after squeezing.

A phosphate coating with a weight per unit area of 0.9 to 1.5 gm⁻² wasproduced on the steel electrolytically galvanized on one side.

                                      TABLE 1                                     __________________________________________________________________________    Composition of phosphating baths                                                                    Comparison                                                            Examples                                                                              Examples                                                              1   2   1       2     3                                         __________________________________________________________________________    Bath parameter                                                                FA.sup.1) (points)                                                                          5.5 4.6 2.2     3.5   4.6                                       TA.sup.2) (points)                                                                          30  40  16      20    37                                        Zn.sup.2+  g/l                                                                              4.2 4.2 2.4     3.0   1.0                                       Mn.sup.2+  g/l                                                                              0.0 1.7 0.0     0.0   1.7                                       Ni.sup.2+  g/l                                                                              2.6 1.3 2.1     2.4   1.6                                       PO.sub.4.sup.3- g/l                                                                         16.2                                                                              18.4                                                                              8.5     10.1  16.7                                      Temp. °C.                                                                            63  55  60      60    63                                        Time s        5   6   6       6     5                                         Weight of phosphate coating                                                   (Zn side) gm.sup.-2                                                                         1.5 1.4 1.5     1.3   0.9                                       Appearance of Metall.                                                                           Metall.                                                                           Serious brown                                                                         Streaky                                                                             Light                                     ungalvanized  bright                                                                            bright                                                                            phosphate                                                                             phosphate                                                                           coating                                   surface               discoloration                                                                         coating                                         __________________________________________________________________________     .sup.1) FA = free acid                                                        .sup.2) TA = total acid                                                  

We claim:
 1. A process for phosphating a galvanized surface of a steelstrip electrolytically galvanized on one side to form anickel-containing phosphate coating which comprises: contacting at leastthe galvanized side of the steel strip for from 2 to 20 seconds at atemperature of 40° C. to 70° C. with a phosphating solution,comprisinga) Zn²⁺ cations: 1.0 to 6.0 g/l, b) Ni²⁺ cations: 0.5 to 5.0g/l, c) PO₄ ³⁻ anions: 14 to 25 g/l, "free acid" content: 3.5 to 8.0points, "total acid" content: 22 to 40 points.to form a nickelcontaining phosphate coating of from 0.5 to 2 gm⁻² whereby anungalvanized surface of the steel strip remains metal bright.
 2. Theprocess as claimed in claim 1, wherein the treatment time is 5 to 10seconds.
 3. The process as claimed in claim 1 wherein the content ofZn²⁺ cations is 3.0 to 5.0 g/l.
 4. The process as claimed in claim 1wherein the content of Ni²⁺ cations is 1.0 to 3.0 g/l.
 5. The process asclaimed in claim 1 wherein the phosphating solution contains 19 to 23g/l of phosphate anions.
 6. The process as claimed in claim 1 whereinthe phosphating solution additionally contains at least one divalentcation selected from the group consisting of manganese, cobalt, iron,calcium and magnesium.
 7. The process as claimed in claim 1 wherein thephosphating solution comprises fluoride anions up to 0.5 g/l.
 8. Theprocess as claimed in claim 1 wherein the phosphating is carried out ata temperature of 55° to 65 ° C.
 9. The process as claimed in claim 1wherein the zinc phosphate coating has a weight per unit area of from1.0 gm⁻² to 2 gm⁻².
 10. The process as claimed in claim 1 wherein thesteel strip, electrolytically galvanized on one side, has been subjectedto an activating pretreatment.
 11. The process of claim 2 wherein thecontent of Zn²⁺ cations is from 3.0 to 5.0 g/l.
 12. The process of claim2 wherein the content of Ni²⁺ cations is 1.0 to 3.0 g/l.
 13. The processof claim 3 wherein the content of Ni²⁺ cations is 1.0 to 3.0 g/l. 14.The process of claim 2 wherein the phosphating solution contains 19 to23 g/l of phosphate anions.
 15. The process of claim 3 wherein thephosphating solution contains 19 to 23 g/l of phosphate anions.
 16. Theprocess of claim 4 wherein the phosphating solution contains 19 to 23g/l of phosphate anions.
 17. The process of claim 1 wherein thephosphating solution further comprises at least one divalent cationselected from the group consisting of manganese, cobalt, iron, calciumand magnesium when, if present, is present in an amount of from 0.2 to3.0 g/l manganese, 0.5 to 4.0 g/l cobalt, 0.05 to 1.0 g/l iron, and 0.5to 5.0 g/l calcium, and 0.5 to 5.0 g/l magnesium.
 18. The process ofclaim 2 wherein the phosphating solution additionally contains at leastone divalent cation selected from the group consisting of manganese,cobalt, iron, calcium and magnesium.
 19. The process of claim 2 whereinthe phosphating solution further comprises at least one divalent cationselected from the group consisting of manganese, cobalt, iron, calciumand magnesium when, if present, is present in an amount of from 0.2 to3.0 g/l manganese, 0.5 to 4.0 g/l cobalt, 0.05 to 1.0 g/l iron, and 0.5to 5.0 g/l calcium, and 0.5 to 5.0 g/l magnesium.
 20. The process ofclaim 3 wherein the phosphating solution additionally contains at leastone divalent cation selected from the group consisting of manganese,cobalt, iron, calcium and magnesium.
 21. The process of claim 3 whereinthe phosphating solution further comprises at least one divalent cationselected from the group consisting of manganese, cobalt, iron, calciumand magnesium when, if present, is present in an amount of from 0.2 to3.0 g/l manganese, 0.5 to 4.0 g/l cobalt, 0.05 to 1.0 g/l iron, and 0.5to 5.0 g/l calcium, and 0.5 to 5.0 g/l magnesium.
 22. The process ofclaim 4 wherein the phosphating solution additionally contains at leastone divalent cation selected from the group consisting of manganese,cobalt, iron, calcium and magnesium.
 23. The process of claim 4 whereinthe phosphating solution further comprises at least one divalent cationselected from the group consisting of manganese, cobalt, iron, calciumand magnesium when, if present, is present in an amount of from 0.2 to3.0 g/l manganese, 0.5 to 4.0 g/l cobalt, 0.05 to 1.0 g/l iron, and 0.5to 5.0 g/l calcium, and 0.5 to 5.0 g/l magnesium.
 24. The process ofclaim 5 wherein the phosphating solution additionally contains at leastone divalent cation selected from the group consisting of manganese,cobalt, iron, calcium and magnesium.
 25. The process of claim 5 whereinthe phosphating solution further comprises at least one divalent cationselected from the group consisting of manganese, cobalt, iron, calciumand magnesium when, if present, is present in an amount of from 0.2 to3.0 g/l manganese, 0.5 to 4.0 g/l cobalt, 0.05 to 1.0 g/l iron, and 0.5to 5.0 g/l calcium, and 0.5 to 5.0 g/l magnesium.
 26. The process ofclaim 2 wherein the phosphating solution comprises fluoride anions up to0.5 g/l.
 27. The process of claim 3 wherein the phosphating solutioncomprises fluoride anions up to 0.5 g/l.
 28. The process of claim 4wherein the phosphating solution comprises fluoride anions up to 0.5g/l.
 29. The process of claim 5 wherein the phosphating solutioncomprises fluoride anions up to 0.5 g/l.
 30. The process of claim 6wherein the phosphating solution comprises fluoride anions up to 0.5g/l.
 31. The process of claim 2 wherein the phosphating is carried outat a temperature of 55° to 65° C.
 32. The process of claim 2 wherein thesteel strip, electrolytically galvanized on one side, has been subjectedto an activating pretreatment,
 33. The process of claim 10 wherein theactivating pretreatment comprises contacting at least theelectrogalvanized surface with a titanium containing activatingcomposition.